Air and Fuel Filter Service Tips

Air and fuel filter maintenance is usually considered easy by most import repair shops, but advancing technology is changing the way it should be performed.

For example, recommended replacement intervals for air filters now range between 15,000 and 75,000 miles and fuel filter replacement intervals vary from ­between 20,000 miles to 150,000 miles, even “lifetime” in some applications. Modern fuel quality is so good that many import manufacturers leave the replacement interval to the technicians’ discretion. In any case, it’s important to compare perception with reality in the modern air and fuel filter markets. See Photo 1.

fuel trims

Photo 1: The positive short-term fuel trims on this data screen can indicate that the fuel filter is clogged or the fuel pump is defective.


Perception and Reality
Despite the advanced engineering invested in modern intake air systems, many technicians continue to believe that an air filter is a necessary restrictive device used to prevent airborne dirt from entering an engine. Many also ­believe that removing an air filter or installing a larger capacity air filter will improve high-speed engine performance and fuel economy.

In reality, modern air filters have more than enough surface area to support air flow during normal driving conditions. Even if air flow through the air filter becomes slightly restricted, the powertrain control module (PCM) will use the mass air flow (MAF) sensor input to match the volume of incoming fuel with the volume of incoming air. So, even while horsepower might be slightly ­reduced due to restricted air flow, fuel economy and power won’t suffer during normal driving conditions.

Many owners and technicians routinely remove, inspect and, perhaps, replace air filters during each oil change interval. A few import manufacturers recommend inspecting an air filter during an oil change, but most recommend inspecting and replacing it at specified intervals. In many cases, frequent inspections allow dirt to fall into the filter housing and the housing is often damaged through repeated disassembly.



Air Filtration Systems
The modern air filtration system not only separates airborne dirt from the inlet air, it must also dampens undesirable intake air noise and, removes turbulence from the incoming air. Modern air filtration systems combine a small cross-sectional area in the air inlet with a larger cross-sectional area in the air filter housing to achieve these goals.

intake air systems

Modern intake air systems such as this remove turbulence from air flowing into the MAF sensor.
The difference in cross-sectional area decreases the velocity of intake air as it enters the air filter housing causing airborne debris to fall to the bottom of the housing. This increases the life of the air filter and tends to dampen the throaty resonance of air rushing into the engine. Last, and most important, this reduction in air velocity helps increase the accuracy of the MAF sensor by removing turbulence from the incoming air.

Unfortunately, many “tuner” or “hot-rod” technicians who replace OE systems with performance ­aftermarket systems might not understand that ­modern MAF sensors basically measure the volume and density of the incoming air by measuring the change in current flowing through a resistor or wire as it is cooled by a smooth, non-turbulent flow of incoming air. This operating principle provides the MAF sensor with the ability to precisely measure air flow in grams-per-second increments.

MAF sensor

Inspecting the MAF sensor for accumulated debris is a major part of scheduled air filter maintenance.
While space doesn’t allow for a full discussion of MAF diagnostics, it’s important to know that dirty MAF-sensing elements miscalculate the amount of air flowing into the engine, which results in an erratic air/fuel ratio calculation and a subsequently erroneous calculated engine load or calculated barometric pressure data value on a scan tool. In most cases, miscalculations caused by dirt and air turbulence actually decrease horsepower and fuel economy.

Of course, the popular perception is that increasing air filter capacity will increase an engine’s horsepower and fuel economy. The reality is that increasing the air flow beyond the capacity of the engine’s peak demand will do nothing to enhance performance. In practically all cases, the OE air filtration system is, by far, the most efficient and the least maintenance-intensive system on the market simply because its design matches the engine’s original intake air flow requirements.



Intake System Service
As I mentioned above, air filter replacements are the most over-performed and least understood of routine maintenance services. But the fact that an air or fuel filter might remain in place for extended intervals should underline the importance of how we perform an air or fuel filter replacement. All too often, inexperienced lube bay employees actually damage the air filter housings when changing a filter or slip an air filter in place without cleaning the air filter housing or sealing surfaces.

Needless to say, dirt leaking past an air filter for thousands of miles will not only shorten engine life, but will foul the MAF sensor, throttle plate and idle air control (IAC) systems with dirt. Even in an era of electronically controlled throttles, dirt accumulation around the throttle plate can change the base air flow enough to cause cold-start and idle speed control complaints.

air filter

Perforations in the pleated areas can be detected by holding a bright light under the air filter media.
When disassembling an air intake system, always inspect the inside of the downstream ducting for dirt contamination from dirt leaking past the air filter. Because dirt around the throttle plate can cause performance complaints, the throttle plate should be cleaned with an aerosol throttle plate cleaner. Never use an aerosol carburetor or brake parts cleaner to clean a throttle body because those solvents might damage throttle shaft seals or peel delicate throttle bore coatings. An old toothbrush can be used to loosen hardened deposits.

Next, inspect the MAF sensor for dirt contamination. Although minor MAF sensor contamination can be cleaned with an aerosol MAF cleaner, remember that cleaning the MAF sensor will usually bring it back to about 80-90-percent efficiency, and a MAF replacement should be considered. Next, lubricate neoprene hose fittings and duct attaching points with a light film of silicone or silicone grease to assist reassembly. Be sure to vacuum all debris out of the air filter housing and clean the sealing surfaces before reassembly. If the ­vehicle has a “snow screen” built into the air inlet, ­inspect and clean if necessary. Replace the housing if it’s damaged or distorted.



Fuel Filter Service
Scheduled fuel filter replacements are often ignored in the quick lube bays because they are more difficult and time-consuming to perform. Many modern vehicles also have the fuel filter built into a “fuel pump module,” which eliminates the need for scheduled replacements. In any case, an excessive load is placed on the electric fuel pump as it struggles to pump fuel through a clogged fuel filter. This is why many fuel pump manufacturers want new filters to be installed with their new fuel pumps.

chassis-mounted fuel filters
Proper precautions should always be taken when replacing chassis-mounted fuel filters. Always lube threaded fuel connections with penetrating oil before removal. It’s also a good diagnostic practice to detect water or rust contamination by draining the filter contents into a clear cup or container.

Next, prevent corrosion by lubricating all threaded fittings and attaching bolts with light grease or an anti-corrosion compound. When inspecting all hoses and clamps for wear and damage, remember that fuel injection systems require special high-pressure fuel hose and clamps. Fuel injection hose clamps are built to avoid cutting or damaging the fuel hose. Fuel injection hose itself is designed to withstand high fuel pressures.

retaining bracket
When reinstalling the fuel filter, make sure that the retaining bracket and fuel connections are firmly ­attached before starting the engine. Incorrect installation techniques can create a fire hazard liability if the fittings or hose attachments leak or become disconnected during normal driving.